The fuel mixtures which, in addition to the actual petroleum fuels, may contain a blend of ethanol and/or other alcohols are being increasingly used in motor vehicles. For example, so-called flex-fuel vehicles are known which may be operated with variable ethanol/gasoline mixtures. The parameters of the engine control of the motor vehicle are typically adapted to the composition of the fuel mixture. The ethanol-fuel mixture ratio is either generally determined based on measuring variables that are present, with the aid of software in the control unit itself, or this mixture ratio may be recognized using an ethanol sensor. Ethanol sensors of this type may be based on many different measuring principles. In particular, capacitance measuring methods based on the permittivity and the conductivity determination are used here. The permittivity of the fuel mixture is generally determined at frequencies of up to approximately 1 MHz.
To identify further components such as water, measurements in the GHz range are necessary, since in this range the permittivity of alcohol, water, and other polar components, for example interfering components, decreases greatly with increasing frequency due to the orientation polarization. One known measuring method is the measurement of an absorption, transmission, or reflection of microwaves by the fuel mixture. An example of such a method which operates in the GHz range is discussed in DE 34 12 704 A1.
In addition, a method is discussed in DE 10 2008 044 403 A1 for determining a composition of a fuel mixture, in which a characteristic pattern of a certain response to microwave irradiation is detected over a fairly great frequency range in order to ascertain the properties of the fuel mixture, and the composition of the fuel mixture is deduced therefrom. One particular advantage of the method discussed in DE 10 2008 044 403 A1 is that the accuracy may be increased by using greater frequency ranges, so that alcohol-fuel mixtures, for example, which additionally contain a water component and/or additives may be characterized much more accurately than with conventional methods.
In measuring methods in the microwave range or high-frequency range, connectors and cables having a coaxial design are usually used for attaching a microwave conductor. In particular, in these types of fuel sensors a sensor electronics system which is provided for at least partial evaluation or signal processing may be situated in the immediate vicinity of the measuring element, and is connected to the coaxial connector of the measuring element in order to couple the detected microwave signal. Due to the high demands on the signal quality and the signal transmission, it is known to use bond connections or solder connections, which at the same time provide necessary thermal decoupling.
However, there is potential for improvement with regard to the coupling of the microwave signal into the sensor electronics system.